Incremental measuring and machine control systems of the type mentioned above may serve to measure angles or lengths. In dependence on the nature of the scale and of the scanning unit, the scale and the reference mark or marks can be scanned by optoelectronic, inductive, magnetic or capacitive methods. In most cases the scanning of the scale results in a generation of basically sinusoidal analog signals, each complete period of which is associated with a complete increment of the scale. For an optoelectronic scanning, each complete scale increment comprises a bright field and a dark field and in the simplest case a graduation line and an empty field or free space. In the present case the term "incremental measuring system" is used to describe also absolute measuring systems having a plurality of coded scale tracks including an incremental scale track having the highest resolution. In most measuring and machine control systems of the present kind, the scanning results in the generation of at least two analog signals, which are displaced in phase, e.g. by 90.degree., and in dependence on the direction of the scanning movement performed by the scanning unit relative to the scale one of said signals or the other leads the other of said signals so that the direction of the scanning movement can be detected from said analog signals by a direction discriminator. In the processor, digital signals which can be counted are derived from the analog signals. This may be accomplished by means of trigger circuits detecting the zero crossings of the phase-displaced analog signals, and circuits for subdividing the scale increments electronically or by computation may be provided and may comprise a microprocessor or microcomputer. The digital signals can be counted and the resulting counts can be used in a display unit for displaying the instantaneous result of the measurement, which may be related to a selected zero point. The digital signals may also be used as control signals or position control signals in a path control system of a machine tool or can be used for monitoring and/or controlling the position of industrial robots.
The provision of one or more reference marks permits an absolute determination of one or more points of reference on the incremental scale. In response to a movement past a reference mark, the scanning unit generates a pulse which is associated with a specific scale increment and which is as long as, or shorter than, a signal that is derived from that scale increment. The reference signal may be used to initiate the operation of the processor or display unit or to set it or to reset it to zero. Under the control of the reference signal, a bidirectional counter of the processor may be set, started or stopped. The reference signal also permits the results of the measurement to be related to absolute points of referene so that the incremental measuring and machine control system operates as a virtually absolute system.
Incremental measuring and machine control systems of the kind described first hereinbefore are known from GB-B-2,065,872 and from DE-A-32 45 357. In these known measuring and machine control systems, a plurality of reference marks are provided in a common auxiliary track of the rule. Control marks, mechanical stops or permanent magnets are provided or can be attached to the rule in another auxiliary track, which is disposed in most cases on that side of the scale which is opposite to the reference marks. By means of said control marks and associated switching means, specific reference marks or reference signals derived from such reference marks can be selected from the large number of reference marks or reference signals which are provided so that only the selected reference signals are subjected to further processing. It is emphasized that in the prior art the control marks are used only to control the processor of the measuring system and do not perform additional functions.
In known path control systems for machine tools, the digital signals derived from the analog signals are utilized for a control. A computer may be used to define memory address ranges corresponding to workpiece portions which are to be machined by means of a tool and said memory address ranges may be related to absolute or selected points of a scale. In such an arrangement, the instantaneous position of the tool can be detected in dependence on the currently received digital signals and said digital signals can be used to adjust the tool to a desired position. In a known control system, a desired position to be reached by the tool is stored in a memory and the path length to be travelled by the tool until that desired position has been reached is computed. That path length to be traversed is also stored in a memory and the count resulting from the counting of the digital signals during the movement of the tool is compared with the count corresponding to the required path length. This operation is continued until the desired position has been reached. At that time the computer generates a signal for initiating the next operation. It is also known in connection with machine tools or robots to provide separate limit switches for defining certain positions of a tool slide, toolholder, carriage or saddle. Such limit switches may consist of mechanically actuated switches or of non-contacting switches, such as photodetectors, proximity switches or magnet-actuated reed relays. The limit switches which are additionally required increase the overall expenditure involved in the machine tool and often can be attached to the machine tool only with great difficulty. In many cases it is almost impossible to attach such limit switches to an existing machine or to alter the locations defined by such switches.
It is an object of the invention to provide an incremental measuring and machine control system which is of the kind described first hereinbefore and in which the measuring system is provided with simple means for controlling the functions of switching means of the machine control system, particularly the functions of limit switches.